1. Field of the Invention
The present invention relates in general to a dielectric ceramic composition used for producing a dielectric resonator or filter for microwave application, and a method of preparing such a dielectric ceramic composition. In particular, the invention is concerned with a dielectric ceramic composition which can be fired at a relatively low temperature, and which is suitably used for dielectric resonators having internal conductive strips, of a stripline type filter, for example, and with a method of preparing such a dielectric ceramic composition. The present invention is also concerned with a dielectric resonator for microwave application, which is obtained by using the above dielectric ceramic composition, or a dielectric filter having a plurality of such dielectric resonators, and with a method of producing the dielectric resonator or dielectric filter.
2. Discussion of the Prior Art
In a modern microwave telecommunication system such as a portable or automobile telephone system, there is widely used a coaxial type dielectric filter using a ceramic composition having a high dielectric constant. The coaxial type dielectric filter has a plurality of coaxial type resonators connected to each other. Each resonator is a cylindrical dielectric block which has inside and outside conductors formed on inner and outer circumferential surfaces of the block, respectively. This type of dielectric filter has a limitation in reducing the size and thickness thereof due to its construction. In view of this, there is proposed a stripline type filter of a tri-plate structure, which incorporates internal conductive strips or layers within a dielectric substrate. In this stripline type filter, a patterned array of conductors in the form of strips are integrally embedded in the dielectric substrate so as to provide a plurality of resonators. The thus constructed stripline type filter is comparatively compact and thin.
In fabricating such a stripline type dielectric filter having the internal conductive strips as described above, a dielectric ceramic composition must be co-fired with the internal conductive strips. Since known dielectric ceramic compositions have a considerably high firing temperature of 1300.degree.-1500.degree. C., there is a limit to conductive materials which can be used for the internal conductive strips, thus making it difficult to employ a Cu-contained or Ag-contained material having a relatively low conductivity resistance. For instance, the firing temperature of the dielectric ceramic composition must not exceed 1100.degree. C. when alloys containing Cu or Ag are used for the internal conductive strips, and must not exceed 1000.degree. C. when Ag--Pd or Ag--Pt alloys are used for the conductive strips. When the conductive layers are formed solely of Ag having a low conductivity resistance, in particular, the firing temperature of the dielectric ceramic composition must be controlled to be around 900.degree. C. which is lower than the melting point of Ag, that is, 962.degree. C. To meet the needs, a dielectric ceramic composition is needed which can be fired at a sufficiently low temperature while assuring excellent microwave characteristics.
Among various dielectric ceramic compositions for microwave application which have been proposed, a dielectric ceramic composition which contains BaO and TiO.sub.2 as major components is known as having a high specific dielectric constant of about 30-40, a large unloaded Q, and a small temperature coefficient of the resonance frequency. In JP-B2-58-20905, for example, there is provided a detailed description of a method of producing a dielectric ceramic of BaO--TiO.sub.2 system, which is composed of Ba.sub.2 Ti.sub.9 O.sub.20 and fired at a temperature as high as 1300.degree.-1400.degree. C. Thus, the known method does not meet the above requirement for a lower firing temperature.
It has been proposed in JP-A-57-69607, JP-A-58-73908 and JP-A-60-257008 that ZnO or SnO.sub.2, or ZnO and ZrO.sub.2 is/are added to the dielectric ceramic composition of the BaO--TiO.sub.2 system, so as to improve the dielectric properties of resulting dielectric ceramics. However, this dielectric ceramic composition still requires an undesirably high firing temperature of 1200.degree.-1400.degree. C.
In one method of producing a temperature-compensating thick-film condenser as disclosed in JP-B2-55-22012, a dielectric ceramic powder of TiO.sub.2 --SnO.sub.2 --BaO system is mixed with a glass frit such as a barium lead borosilicate glass and a lead borosilicate glass, and the mixture obtained is fired at 950.degree. C. to provide a dielectric ceramic. While the thus obtained dielectric ceramic may be suitably used for thick-film condensers, a dielectric resonator or filter for microwave application, which is produced by using the dielectric ceramic, exhibits a deteriorated unloaded Q and an increased temperature coefficient of the resonance frequency, which adversely affect the function of the resonator or filter.